1. Field of the Invention
The present invention relates generally to hydraulic booster units and more particularly to a hydraulic brake booster unit adapted for use in a vehicle.
2. Description of the Prior Art
Within the prior art boosters, especially of the closed center type, fluid from a power source has been permitted to flow into a booster housing through means of an annular grooved chamber provided at the outer peripheral surface of a power piston. One such conventional construction is illustrated within U.S. Pat. No. 2,883,971 wherein a high pressure fluid is fed from an accumulator 34 into an annular chamber 32 provided at the outer periphery of a power piston 28, and further to a normally closed valve assembly 61 and 54, as shown within FIG. 1. This method of introducing fluid into a booster unit, however, has a serious drawback.
As the power piston 28 has seal members 30 for sealing the high pressure within the annular chamber 32 of the power piston 28, these seals 30, two being shown, must have a diameter approximately the same as that of the power piston, especially that of the piston body 29, and accordingly, the cross-sectional diameter thereof must be increased relative to the whole diameter thereof for attaining sufficient sealing. Since the piston 28 is slidable or reciprocable, the cross-sectional diameter of seals 30 must, however, be reduced as much as possible in order to minimize the sliding resistance force exerted upon the piston 28 for smooth sliding of the same.
Another prior art booster is disclosed within U.S. Pat. No. 3,792,641, has apparently solved such sealing problems. A high pressure fluid from a pump is permitted to flow into one side of a closed valve 46 through means of an annular groove 22 provided at the outer peripheral surface of an input piston 20. Since the diameter of the input piston 20 is smaller than that of a power piston 68, the diameter of the seals upon the input piston 20 may be, in this sense, reduced by a certain amount. However, the sliding resistance force exerted upon the input piston 20 is directly transmitted to a pedal 106, that is, the whole sliding resistance force is transmitted to a pedal, and the operator, as the resistance force.